We report the first observation of coherent vibrational dynamics in a hydrogen bond obtained by femtosecond nonlinear measurements in the mid-infrared spectral range. The results provide the first experimental evidence in the time domain for the anharmonic coupling of slow and fast vibrational motions in a hydrogen bond. We show that the absorption band of the high-frequency hydrogen stretching (O-H/O-D) vibration is modulated by a coherent oscillatory motion corresponding to a periodic variation of the hydrogen bond length and its strength. This mechanism,, which is of general relevance for hydrogen bonds, is connected with underdamped low-frequency vibrations-in contrast to the assumption of overdamped nuclear motions in most theoretical treatments.